   "Rename objects in ctx by given hash old-to-new."
   [ctx old-to-new]
   (let [objs (map old-to-new (objects ctx))
-	inz (set-of [(old-to-new g) m] [[g m] (incidence ctx)])]
+        inz (set-of [(old-to-new g) m] [[g m] (incidence ctx)])]
     (make-context-nc objs (attributes ctx) inz)))
 
 (defn rename-attributes
   "Rename attributes in ctx by given hash old-to-new."
   [ctx old-to-new]
   (let [atts (map old-to-new (attributes ctx))
-	inz (set-of [g (old-to-new m)] [[g m] (incidence ctx)])]
+        inz (set-of [g (old-to-new m)] [[g m] (incidence ctx)])]
     (make-context-nc (objects ctx) atts inz)))
 
 (defn make-context-from-matrix

  \\]"
  [ctx-1 ctx-2]
  (let [new-objs (cross-product (objects ctx-1) (objects ctx-2))
	new-atts (cross-product (attributes ctx-1) (attributes ctx-2))
	inz-1    (incidence ctx-1)
	inz-2    (incidence ctx-2)
	new-inz  (set-of [[g_1, g_2], [m_1, m_2]]
			 [[g_1, g_2] new-objs
			  [m_1, m_2] new-atts
			  :when (or (inz-1 [g_1, m_1])
				    (inz-2 [g_2, m_2]))])]
    (make-context-nc new-objs new-atts new-inz)))

(defn context-semiproduct

  "Computes the down arrow relation of ctx."
  [ctx]
  (let [obj   (objects ctx),
        att   (attributes ctx),
        inz   (incidence ctx),
        prime (map-by-fn #(object-derivation ctx #{%}) obj)]
    (set-of [g m]
            [g obj
             m att
             :when (and (not (inz [g m]))
                        (forall [h obj]
                          (=> (proper-subset? (prime g) (prime h))
                              (inz [h m]))))])))

(defn up-arrows
  "Computes the up arrow relation of ctx."
  [ctx]
  (let [obj   (objects ctx),
        att   (attributes ctx),
        inz   (incidence ctx),
        prime (map-by-fn #(attribute-derivation ctx #{%}) att)]
    (set-of [g m]
            [g obj,
             m att
             :when (and (not (inz [g m]))
                        (forall [n att]
                          (=> (proper-subset? (prime m) (prime n))
                              (inz [g n]))))])))

(defn up-down-arrows
  "Returns up-down-arrow relation of ctx."
